(Mishnah): Ben Sorer u'Moreh - from when can a boy can become a Ben Sorer u'Moreh? It is from when he brings two hairs (after 13 years) until his beard fills out. This refers to the lower (i.e. pubic) beard, and not the upper one. Chachamim use a clean expression.

We learn from "Ki Yihyeh l'Ish Ben" - a son, and not a daughter; a son, and not an adult;

A minor is exempt for he did not yet come to [obligation to fulfill] Mitzvos.

(Gemara - R. Ze'ira citing R. Avahu citing R. Yosi ben Chaninah citing Reish Lakish): It says "v'Chi Yazid Ish Al Re'ehu Lehorgo v'Armah." From when does he become an Ish (and cannot be a Ben Sorer u'Moreh)? It is from when 'cooks' [his seed, and enables it to produce a child]. From when does he 'cook'? It is from when the Kaf (the area above the Ever and below the stomach) spreads.

A parable - if one cooks seeds inside, the pot blackens outside. (The Kaf blackens through growth of hair).

(R. Zeira taught in the name of R. Shila bar Bina): "Ki Yihyeh l'Ish Ben", and not that the son is a father. Once he is proper to have Bi'ah with a woman and impregnate her, [after three months the fetus can show, and it is seen that] he is a father, and not [only] a son, and the Torah said a son, and not a father.

This is like R. Yosa said in the name of R. Shabsi. All the days [proper to become] a Ben Sorer u'Moreh are only three months.